The present invention relates to a method for reclaiming waste sulfuric acid discharged from, for instance, a titanium oxide-production process.
The so-called sulfuric acid method has been well-known as a method for preparing titanium oxide (titania) widely used as a coloring material. A large amount of waste sulfuric acid is discharged mainly from the solid/liquid separation step for titanium hydroxide during the titanium oxide-production process. The waste sulfuric acid contains, for instance, Fe, Ti, Cr and V originated from ilmenite and titanium slug as raw materials for titanium.
In the past, the waste sulfuric acid was neutralized and then burried under the ground or dumped at sea in the form of gypsum. However, new techniques for treating the waste sulfuric acid are required because of various problems, for instance, environmental pollution, the cost of insurance for the land in which gypsum is buried and the enormous expenses necessary for the post-treatment of the waste sulfuric acid.
There have been a variety of proposals in response to these problems. For instance, Japanese Un-examined Patent Publication (hereunder referred to as xe2x80x9cJ.P. KOKAIxe2x80x9d) Nos. Hei 3-80103 and Hei 3-88718 each discloses a method for removing impurities such as Fe ions, which makes use of a solvent-extraction process. In these methods, however, the strongly acidic sulfuric acid solution per se is directly subjected to a solvent-extraction treatment. Therefore, Fe ions are, in fact, extracted in poor efficiency and the iron-containing component thus recovered also has low purity, in spite of the assertions of the inventors of the foregoing patents. Moreover, in these methods, trivalent iron ions are directly extracted so the use of an acidic extraction agent would require a multistage extraction treatment because of the low extraction rate thereof.
Under such circumstances, the inventors of this invention have developed a method for effectively recovering a highly concentrated sulfuric acid solution and filed a patent application, EP-A1-0541002, in which the aforementioned solvent-extraction method is improved so as to enhance the bivalent iron ion-removing efficiency. However, the sulfuric acid solution thus recovered is dark green because of the presence of residual Cr and V which are converted into colored oxides and it is difficult to reuse the colored sulfuric acid solution in the process for preparing titanium oxide used, in particular, as a white pigment.
Moreover, there has also been proposed a diffusive dialysis method, as a technique for treating the waste sulfuric acid, which makes use of an anion exchange membrane. However, if the waste sulfuric acid is treated through the diffusive dialysis, particles are deposited on the surface of the ion exchange membrane immediately after putting the dialyzer in operation and this leads to deterioration of the quality of the membrane and breakage of the membrane due to deposition of particles within the membrane. For this reason, if the foregoing phenomenon occurs during the operation of the device over a long period of time, the deposits should be removed by washing the anionic exchange membrane with an agent capable of washing them away.
To eliminate the problem of poor efficiency, J.P. KOKAI No. Hei 2-153806 proposes a method for recovering sulfuric acid present in a titanium-containing waste sulfuric acid solution while setting the solution against water in a multistage diffusive dialysis tank equipped with an anion exchange membrane wherein the solution and water are countercurrently supplied to the dialysis tank. In other words, the former is passed, through the tank, from the front stage to the rear stage, while the latter is passed therethrough from the rear stage to the front stage.
This method permits the elimination of the step for washing the anion exchange membrane and a decrease in the degree of quality-deterioration of the membrane, but the membrane should be washed every 10 days as is described in Examples of the patent. Therefore, the method should further be improved in its efficiency.
It is an object of the present invention to solve the foregoing problems and to provide a method for efficiently and easily regenerating a sulfuric acid solution having a high concentration and a low impurity content from metal sulfate-containing waste sulfuric acid and waste liquor formed during acid cleaning which are discharged from the process for preparing titanium (IV) oxide according to the sulfuric acid method.
According to the present invention, the foregoing object can be accomplished by removing titanium present in a metal sulfate-containing waste sulfuric acid solution through a solvent-extraction treatment and then subjecting the solution, from which titanium is extracted, to a diffusive dialysis treatment.
It is preferred to backwardly extract the titanium thus extracted by bringing the organic solvent, which is obtained through the solvent-extraction and contains titanium, into contact with an alkaline solution to thus convert the titanium into an insoluble compound through the backward extraction; to bring the organic solvent obtained after the removal of titanium into contact with the remaining solution obtained after the diffusive dialysis treatment for regenerating the organic solvent through the backward extraction of the alkali ions into the aqueous phase; and to reuse the regenerated organic solvent in the solvent-extraction process for reclaiming the waste sulfuric acid.
Moreover, it is desirable that the waste sulfuric acid solution be optionally filtered while cooling to control the concentration of free sulfuric acid present therein prior to the solvent-extraction of the solution.
The method for reclaiming metal sulfate-containing waste sulfuric acid according to the present invention basically comprises the step of effectively removing principal impurities such as iron, titanium, chromium and vanadium to thus reclaim a solution of highly concentrated sulfuric acid which can be widley reused.
Among these impurities, titanium ions are included in, for instance, the waste sulfuric acid solution discharged from the titanium oxide-production process in an amount ranging from 2 to 10 g/l (these ions are dissolved therein mainly in the form of TiO-SO4 and partially in the form of Ti2(SO4)3). Easily hydrolyzable tetravalent titanium ions, among others, can form titanium oxide particles which may cause quality-deterioration and breakage of the ion exchange membrane from deposition thereof on the surface of the ion exchange membrane and within the membrane during the diffusive dialysis treatment. For this reason, removal of these ions is indispensable for industrially applying the diffusive dialysis method to the reclamation of the metal sulfate-containing waste sulfuric acid. The inventors of this invention have found out that the formation of titanium oxide particles on an ion exchange membrane can substantially be inhibited if the titanium content in the solution is, in advance, reduced to a level of not more than 0.5 g/l through solvent-extraction. The titanium components remaining even after the solvent-extraction are mainly composed of trivalent titanium ions which are less hydrolyzable as compared with tetravalent titanium ions. Therefore, the waste solution obtained after the solvent-extraction treatment may directly be subjected to a diffusive dialysis treatment without impairing the intended effect of the method to yield a sulfuric acid solution having a low content of impurities such as chromium and iron. In particular, it is desirable to remove solid contents present in the waste solution obtained after the solvent-extraction through micro filtration prior to the diffusive dialysis thereof. Moreover, if the organic solvent still remains in the waste solution, it is desirable to remove the remaining organic solvent by bringing the waste solution into contact with, for instance, activated carbon.
If the method of the present invention is used for reclaiming waste sulfuric acid, the method can be carried out according to the batchwise process in which a complete waste solution as a waste acid solution is treated. The method can also be used in or the so-called continuous recovery process which comprises the steps of continuously withdrawing a waste sulfuric acid solution from, for instance, a titanium oxide-production process, recovering the acid and simultaneously separating and removing metal salts separated from the solution. The continuous recovery process is generally preferred, since the organic solvent used in the solvent-extraction treatment can be regenerated by removing titanium components through backward extraction with an alkali solution.
The extraction of titanium ions is carried out using, for instance, an acidic organophosphoric acid compound, a carboxylic acid, a sulfonic acid or hydroxy oxime, as has been well-known in the art. Among the acidic organophosphoric acid compounds, acidic phosphoric acid diesters are particularly preferred.
Titanium ions extracted with, for instance, an acidic organophosphoric acid compound, are backwardly extracted with, for instance, an alkali solution such as an ammonium carbonate solution and recovered in the form of, for instance, hydroxide. The titanium compound can be fired to give titanium oxide after the recovery thereof. The product can be used as a raw material for titanium.
Ferrous sulfate can be crystallized and the sulfuric acid concentration of the waste solution can be controlled to a level favorable for the diffusive dialysis treatment by filtering the waste sulfuric acid solution while cooling prior to the solvent- extraction treatment thereof.
In the method of the present invention, titanium components present in the metal sulfate-containing waste sulfuric acid solution are removed through the solvent-extraction treatment prior to diffusive dialysis treatment of the waste solution. Thus, the waste sulfuric acid solution can quite efficiently and easily be reclaimed to yield a sulfuric acid solution having a low content of impurities such as iron. In contrast the conventional diffusive dialysis method is inefficient. Moreover, the diffusive dialysis method permits the removal of components such as chromium and vanadium which cannot be removed by the solvent-extraction method. The removal of such components is quite beneficial from the viewpoint of environmental sanitation.
In particular, the method of the present invention has great industrial value. This is because the method permits effective recovery of sulfuric acid from a metal sulfate-containing waste sulfuric acid solution in a closed system and the reuse thereof and also permits the prevention of any environmental pollution by bringing an organic solvent, containing extracted titanium and obtained through a solvent-extraction treatment, into contact with an equal amount of an alkali solution to thus backwardly extract titanium and to convert it into an insoluble compound: bringing the organic solvent obtained after the titanium extraction into contact with the liquid remaining after the dialysis formed during a diffusive dialysis treatment to thus regenerate the organic solvent through backward extraction of alkali ions into the aqueous phase; and using the regenerated organic solvent in the solvent-extraction process for reclaiming the waste sulfuric acid solution. In addition, titanium, scandium or the like can be recovered as by-products and, therefore, the method of the present invention is likewise quite beneficial from the viewpoint of effective use of resources.
If the waste sulfuric acid solution is filtered while cooling prior to the solvent-extraction, iron sulfate can easily be obtained as well.